The present invention relates to a moving target indication radar and, more particularly, to a radar of this kind so designed for use as an ASR (airport surveillance radar) or an ARSR (air route surveillance radar) that can display the returns from moving targets including those of close-to-zero relative radial speeds while eliminating not only the ground clutter attributed to buildings and undulating terrains but also the moving clutter caused by sea surface, large flocks of birds and rain clouds.
Although the radar of this invention is not limited in its application to air traffic control, the following description will be given in conjunction mainly with this field of use.
The conventional radar signal processing technique, known as MTI (moving target indication), for eliminating from a radar signal the returns from stationary objects to detect only the moving target returns for display has been in extensive use. An MTI canceller is made of a delay line and a subtractor combined to give the so-called comb filter characteristic, which is effective to eliminate the ground clutter having close-to-zero frequency components.
However, it is impossible for the MTI canceller to detect such a target as an aircraft flying tangentially to the radar and consequently having zero or close-to-zero Doppler speed with respect thereto, because the returns from such target are totally eliminted together with the returns from the stationary objects, i.e., ground clutter. In other words, the conventional MTI canceller inevitably involves the problem of blind speed.
Another problem unavoidable with the MTI canceller is that the moving clutter having Doppler speed components can never be eliminated. In a radar system utilizing the MTI canceller, therefore, the desired moving target indication is obscured by the moving clutter. A typical example of such moving objects causing the moving clutter is the so-called sea clutter caused by waves. The sea clutter, which ordinarily has a Doppler speed of a few meters per second, spreads as extensively as the ground clutter and greatly affect the quality of the desired indication of moving targets.
Besides the sea clutter, the moving clutter includes the so-called "angel echo" caused by large flocks of migrating birds, and the weather clutter caused by rainfall and rain clouds. Since the area of radar application most vulnerable to these ground clutter lies in air traffic control, there has been a great demand for an improved clutter elimination technique to secure the safety of the ever-increasing air traffic.
Among the conventional techniques designed to meet this requirement, Log-CFAR (Logarithmic Amplification and Constant False Alarm Rate) technique is based on the fact that the sea and weather clutter has an amplitude distribution similar to the Rayleigh distribution. The Log-CFAR technique employing the combination of a logarithmic amplifier and a CFAR circuit is capable of suppressing the clutter components to a level comparable to the noise level inherent to the radar receiver. (For further details, reference is made to a paper entitled "Detection Performance of the Cell Averaging Log/CFAR Receiver" by V. G. Hansen and H. R. Ward, IEEE Trans. of AES, AES-8, p. 648, 1972). However, the desired target detection is impossible for the Log-CFAR technique when the target returns are not higher in level than the moving clutter.
Another example of the conventional clutter eliminating technique is the so-called clutter locking. (For details, reference is made to F. E. Natherson, Radar Design Principles, p. 327-328, McGraw Hill, 1969). However, a sufficient clutter elimination cannot be expected with this technique if there are two different kinds of clutter involved significantly differing in Doppler frequency. Furthermore, even high-level target returns can be eliminated if there is virtually no clutter in the scanned space.
These difficulties involved in these conventional Log-CFAR and clutter locking techniques are attributed to the signal processing performed in the time domain. For the same reason, the separate recognition of two or more targets are impossible if they are in the same range from the radar and differ in Doppler speed.
To solve these problems, the signal processing in the frequency domain has come to be used. For this purpose, the radar signal must be converted through Fourier transform to various clutter and target components mutually separated in frequency domain. The Fourier transform and the subsequent signal processing must be performed on the real time basis. The recent progress in LSI technology has made it possible to achieve the fast Fourier transform resorting to the algorithm proposed by J. W. Cooley and J. W. Tukey. (For details, reference is made to "An Algorithm for the Machine Calculation of Complex Fourier Series," Mathematics Computation, p. 297-301, April 1965). Under the circumstances, further progess in the application of the real-time radar signal processing can be expected.
However, the mere conversion of a radar signal to several Doppler frequency groups does not make the detection of target returns possible.
To overcome this difficulty, C. E. Muehe and his collaborators have proposed an MTD (moving target detector) capable of signal processing in the frequency domain to detect targets in the presence of various clutter. (For details, reference is made to C. E. Muehe et al "Digital Signal Processor for Air Traffic Control Radars," IEEE NEREM'74 Proceedings, p. 28-31, October 1974). The MTD, which is totally digitized for signal processing, employs two separate signal processing means, one for the ground clutter elimination and the other for the moving clutter elimination. More specifically, the radar signal received from the scanned space is converted first to a digital signal and applied through a buffer memory to an MTI canceller and an 8-point DFT (discrete Fourier transform) circuit, where the conversion of the digitized radar signal to eight Doppler frequency groups is performed. On the other hand, the core memory output is supplied, through a zero-Doppler speed component detector provided in parallel with the serially connected MTI canceller and the 8-point DFT and a recursive filter, to a magnetic disc memory to form a "clutter map". Also, the eight Doppler frequency group outputs from the 8-point DFT circuit are subjected to weighting between the every two adjacent frequency components and then to the range-correlation processing, respectively, for a clutter-free display. The range-correlation processing or the range averaging is performed with respect to a predetermined radial distance. The range-correlation processing is performed by the averaging of the radar return levels with respect to a plurality of unit radial regions covering the predetermined radial distance. Since a target has a limited length in the radial direction, the target return can be detected by the threshold control performed depending on the average radar return level obtained with respect to the mentioned radial regions with the unit region under processing lying in their center. In contrast, since the clutter generally has a wider radial spread and contributes more greatly to the average value, it is eliminated by the mentioned averaging and the threshold control.
Meanwhile, the magnetic disc memory stores the zero-Doppler speed components for each of the unit azimuth regions. The magnetic disc memory is adapted to store the zero-Doppler speed components on a single recording track for every coherent processing interval (CPI) equal to ten radar pulse repetition period. Also, one-revolution sweep (360.degree.) is divided into 480 CPI's, with every CPI accommodating the radar data for 768 unit range regions. It follows therefore that the disc memory must have 480 .times. 768 (= 368,640) unit memory regions. Since 10 bits are assigned to each of the unit range region radar data in the MTD, the magnetic disc has a 480 .times. 768 .times. 10 (= 3,686,400)-bit memory capacity. Owing to the recursive filter replacing 1/8 of the memory content for every radar scanning pulse, the stored data represent the time averaged zero-Doppler spaced data for all the unit azimuth -- range regions. The stored data is accessed every four memory tracks (4 CPI's) to permit the averaging among the readout data.
As described, the readout of the stored data synchronized with the scanning by the radar pulse covers an azimuthal region corresponding to 4 CPI's or 40 pulse repetition periods thereby to permit the averaged readout data to eliminate the clutter components through the threshold control. Also, the time-averaging combined with the correlation processing in the azimuthal direction is performed for every unit range-azimuth azimuth region, making it possible to detect a moving target flying tangentially or having a close-to-zero Doppler speed.
As stated above, the MTD has provided one solution to the above-mentioned problems unavoidable with the conventional clutter elimination techniques. However, it requires such a large capacity memory equipment as the magnetic disc. Furthermore, the problem unavoidably coming from the use of the MTI canceller, i.e., the deterioration of the S/N ratio for target returns from a clutter-free space, cannot be solved with the MTD. (For further details, reference is made to G. M. Dillard "Signal-to-Noise Ratio Loss in an MTI Cascaded with Coherent Integration Filters," The Record of IEEE 1975 International Radar Conference, p. 117-122, April 1975).